Optical comparators project images of objects under inspection onto display screens for comparison against a reference datum. Comparisons to the reference datum can be made in association with different types of illumination, including direct lighting, back lighting, and oblique lighting of the objects.
The comparators include optical imaging systems responsible for projecting the images of objects under inspection onto display screens. Objectives can be used to form intermediate images of the objects, and the intermediate images can be magnified by projectors for producing display images capable of comparison to the reference datum.
So called “through-the-lens” illuminating systems have been used for illuminating the objects under inspection by directing light through the objectives to the objects. The illuminating light is generally produced by light sources located remote from the optical imaging systems but producing light beams that generally intersect light paths through the optical imaging system. Inclined mirrors of such illuminating systems surrounding aperture stops of the illuminating systems fold the illuminating beams into alignment with the light paths of the imaging systems.
Light sources of the type used for such illuminating systems, such as mercury arc lamps, tend to be large and expensive, and can raise safety concerns. Some countries, for example, have banned mercury arc lamps. The invention includes among its objects the replacement of such large light sources while providing through-the-lens illumination of objects under inspection.